1. Field of the Invention
The present invention relates to a spark plug for internal combustion engines used for vehicles, co-generation systems, gas transfer pumps or the like.
2. Description of the Related Art
A spark plug for internal combustion engines installed in vehicles, co-generation systems or the like has used in severe environments so that the spark plug requires high airtight structure and strength that resists severe oscillations under elevated temperatures.
As an example of the spark plug, there is disclosed a spark plug in Japanese Utility Model No. S64-2384. In the spark plug disclosed in the Utility Model, an annular space portion is disposed between a housing and an insulator, and powder filling is filled in the annular space portion to form a filled portion for improving airtightness of the spark plug. In addition, in the spark plug, a sleeve of the housing is caulked.
In the spark plug, the filled portion in which the powder filling is filled compensates difference of thermal expansion between the housing and the insulator to keep the airtightness therebetween and hold the insulator.
As conventional powder filling, granulated powder grains are used. The granulated powder grains are manufactured as follows. That is, raw powder particles are prepared and organic component such as binder solution is added to the raw powder particles. The raw powder particles to which the binder solution is added are mixed with each other to be granulated and sized so that the granulated powder grains are manufactured.
For example, the raw powder particles each of which has 50 μm or less in grain diameter are granulated in the above manner so that granulated powder grains each having 100 μm and over in grain diameter are manufactured.
However, in the conventional spark plug, when using the conventional spark plug under environmental conditions with elevated temperatures and severe oscillations, the airtightness and the strength of each of the granulated powder grains are decreased with time so that it may be difficult that the conventional spark plug keeps the function in itself.
That is, in the conventional spark plug, the granulated powder grains are manufactured by mixing the raw powder particles and the organic component such as the binder solution. Each of the granulated powder grains involves air at the granulating process so as to have a wholly porous shape.
Therefore, even when pressing the powder filling each grain of which has a wholly porous shape to be filled in the annular space portion, the filled portion has a plurality of gaps between each of the grains of the powder filling, causing the airtightness of the spark plug to be easily decreased, and the mechanical strength thereof to be easily weakened.
In addition, because of adding the organic component such as binder solution to the raw powder particles so as to manufacture the powder filling including the organic component, in a case of using the spark plug including the powder filling composed of granulated powder grains under elevated temperatures for a long period, the organic component in the powder filling volatilizes, causing pore rate in the powder filling to be increased.
The increase of the pore rate in the powder filling causes the airtightness of the filled portion to be decreased with time and/or the mechanical strength thereof to be weakened with time.
The holding strength of the filled portion with respect to the insulator, filled portion that is composed of the powder filling having low density, is decreased so that, when installing the spark plug in an engine, a top portion of the insulator may be pried with a plug wrench, causing a crack of the insulator.